the best way to think of the internal combustion engine is as an air pump...the more air you can put through the engine the more powerful it becomes...i.e. an intake header and exhaust help free up stock restrictions free up hp lost

in fact you can't make more hp with an exhaust...you can only free up hp that is lost in a restrictive exhaust...honda exhasts tend to be restrictive b/c of emissions requirements

an aftermarket header typically has a wider opening allowing air to escape the engine easier...it also has polished surfaces helping get rid of the stock rough surfaces that create friction that disrupt air flow out of the engine

overall the aftermarket header and downpipe together should offer better exhaust evacuation than the stock unit

there are also several different types of header/downpipe combos on the market ranging from 4-2-1 piece units (4 pipes into 2 then into 1...2 piece header picture is a 4-2-1) and 4-1 units (the one piece header below is a 4-1 unit)...even turbo'd engines have a different header (shorter to accept the turbocharger)

then theres the stainless steel, ceramic coating, protective coating and el cheapo steel exhausts (no protective coating)...this is in regards to how long they last (resist rust)...typically in terms of lasting effect against heat and rust the stainless > ceramic > protective coating > el cheapo headers/downpipe combos

i recommend a dc sports 4-2-1 header...easy to work on and frees up some decent hp

i can also go on about exhaust backpressure and why its bad to eliminate it completely...but i'm tired

NOTE:...i forgot to mention it but there are one piece headers that you can also buy...they offer marginally better hp but at the expense of working on the car if you don't have access to a shop...such as this one